Breast cancer will constitute greater than 30% of all new cancers diagnosed in women this year. Steroid hormones, such as estrogen and progesterone, play an important role in the development and treatment of breast cancer. These receptors represent critical sensors of environmental exposures on reproduction and development while being subject to epigenetic regulation of their activity. Despite intensive efforts, the mortality resulting from this disease has not decreased significantly over the last decade. In my laboratory, the scientists have undertaken detailed analysis of the mechanism of action of the steroid receptors and clinically important steroid receptor antagonists which are used to block their action. Our experiments are focused on the role of chromatin and epigenetics that are critical to understanding their function. It is hoped that these basic research studies will provide new insight into the role that steroid hormone receptors play in breast cancer and the possible development of novel and effective treatments. Our efforts are informed by the overwhelming evidence that a full understanding of transcriptional control requires an appreciation for roles played by the chromatin structure of target genes and the molecular machines that are required to unleash the regulatory potential of steroid receptors. To achieve this we have focused our attention on the mammalian BRG1 chromatin remodeling complex and the 26S proteasome, their interactions with and regulation by the glucocorticoid and progesterone receptors. The activity of these complexes has been evaluated in the context of the chromatin within human and mouse cells with particular attention to the activity of RNA polymerase and the acetylation/methylation of the core histones. Three examples of our efforts are described below: Lymphatic vasculature is an important part of the cardiovascular system with multiple functions, including regulation of the return of interstitial fluid (lymph) to the bloodstream, immune responses, and fat absorption. Consequently, lymphatic vasculature defects are involved in many pathological processes, including tumor metastasis and lymphedema. BRG1 is an important player in the developmental window when the lymphatic system is initiated. In the current study, we used tamoxifen inducible Rosa26CreERT2-BRG1floxed/floxed mice that allowed temporal analysis of the impact of BRG1 inactivation in the embryo. The BRG1floxed/floxed/Cre-TM embryos exhibited edema and hemorrhage at embryonic day-13 and began to die. BRG1 deficient embryos had abnormal lymphatic sac linings with fewer LYVE1 positive lymphatic endothelial cells. Indeed, loss of BRG1 attenuated expression of a subset of lymphatic genes in-vivo. Furthermore, BRG1 binds at the promoters of COUP-TFII and LYVE1, suggesting that BRG1 modulates expression of these genes in the developing embryos. Conversely, re-expression of BRG1 in cells lacking endogenous BRG1 resulted in induction of lymphatic gene expression in-vitro, suggesting that BRG1 was both required and sufficient for lymphatic gene expression. These studies provide important insights into intrinsic regulation of BRG1-mediated lymphatic-gene expression, and further an understanding of lymphatic gene dysregulation in lymphedema and other disease conditions. The Glucocorticoid Receptor (GR) alters transcriptional activity in response to hormones by interacting with chromatin at GR binding sites (GBSs) throughout the genome. Our work in human breast cancer cells identifies three classes of GBSs with distinct epigenetic characteristics and reveals that BRG1 interacts with GBSs prior to hormone exposure. The GBSs pre-occupied by BRG1 are more accessible and transcriptionally active than other GBSs. BRG1 is required for a proper and robust transcriptional hormone response and knockdown of BRG1 blocks recruitment of the pioneer factors FOXA1 and GATA3 to GBSs. Finally, GR interaction with FOXA1 and GATA3 binding sites was restricted to sites pre-bound by BRG1. These findings demonstrate that BRG1 establishes specialized chromatin environments that define multiple classes of GBS. This in turn predicts that GR and other transcriptional activators function via multiple distinct chromatin-based mechanisms to modulate the transcriptional response. Epigenetic enzymes regulate higher-order chromatin architecture and cell-type specific gene expression. The ATPase BRG1 and the SWI/SNF chromatin remodeling complex are epigenetic enzymes that regulate chromatin accessibility during steady and transitional cell states. Experiments in mice show that the loss of BRG1 inhibits cellular reprogramming, while studies using human cells demonstrate that the overexpression of BRG1 enhances reprogramming. We hypothesized that the variation of SWI/SNF subunit expression in the human population would contribute to variability in the efficiency of induced pluripotent stem cells (iPSC) reprogramming. To examine the impact of an individuals, sex, ancestry, and age on iPSC reprogramming, we created a novel sex and ancestry balanced cohort of 240 iPSC lines derived from human dermal fibroblasts (DF) from 80 healthy donors. We methodically assessed the reprogramming efficiency of each DF line and then quantified the individual and demographic-specific variations in SWI/SNF chromatin remodeling proteins and mRNA expression. We identified that BRG1, BAF155, and BAF60a expression as strongly correlating with iPSC reprogramming efficiency. Additionally, we discovered that high efficiency iPSC reprograming is negatively correlated with donor age, positively correlated with African American descent, and uncorrelated with donor sex. These results show the variations in chromatin remodeling protein expression have a strong impact on iPSC reprogramming. Additionally, our cohort is unique in its large size, diversity, and focus on healthy donors. Consequently, this cohort can be a vital tool for researchers seeking to validate observational results from human population studies and perform detailed mechanistic studies in a controlled cell culture environment. Together these studies allow us to fulfill the mission of the NIEHS to improve an understanding of environmental impact on human health and development.